Expanded rubber materials (also known as foam rubber or sponge rubber materials) are materials formed from a matrix of rubber filled with air in pockets and/or channels. In closed cell expanded rubber materials, the air is contained in discrete pockets, whilst open cell expanded rubber materials comprise interconnected air-filled cells. As used herein, the term “expanded rubber” refers to both open cell and closed cell expanded rubber materials; the term “solid rubber” refers to (at least partially cured) rubber materials that have not been expanded to form an air-filled matrix.
Expanded rubber articles are typically significantly lighter than solid rubber articles of the same volume, and may, for example, be at least 90% lighter. For example, if a natural rubber (NR) part has a specific gravity (SG) of 1.2, an equivalent expanded rubber part typically has an SG of 0.12 or less. A further advantage of expanded rubber articles is that they commonly consist of less rubber material than an equivalently sized solid rubber article, and so may, for example, be less costly to produce. In the automotive and aeronautical industries in particular, reducing weight is especially useful for reducing fuel consumption of vehicles and craft.
Another advantage of expanded rubber materials is that they may have a shore hardness as low as 5 shore A, whilst the minimum shore hardness of solid rubber materials is usually around 30 shore A (as measured using an Instron™ hand-held hardness gauge, Shore A type, according to the ASTM D2240 test method). The softer expanded rubber material offers sealing properties not normally found with solid rubber materials because the expanded rubber is more deformable and thus forms better seals around other objects. In particular, an expanded rubber part may deflect at lower loadings than an equivalently sized solid rubber part because the modulus (which typically decreases with increasing ‘softness’) of expanded rubber is considerably lower than that of solid rubber. The (open/closed) microcellular structure of expanded rubber confers a lower modulus to the bulk material than the solid structure of non-expanded rubber.
In comparison to solid rubber materials, expanded rubber materials may, for example, offer improved vibration isolation and vibration damping, anti-shock and noise insulation. Expanded rubber materials can be formulated to offer a range of vibration/noise isolation and damping properties. For example, the hysteresis properties of the expanded rubber material can be controlled by including in the expandable rubber formulation different rubber polymers, fillers, process aids and curing systems.
Typically, expanded rubber materials are prepared by the following process. A composition comprising an elastomeric material, a curing agent and an expansion agent are placed in a mould cavity. The composition is heated to activate the curing agent and the expansion agent to cure and expand the composition to fill the mould cavity with an expanded rubber article. Commonly, the mould cavity is sized to confine expansion of the composition, and so a significant pressure builds up inside the mould. Typically, when the mould is opened, the expanded rubber article ‘jumps out’ and rapidly expands to a size greater than that of the mould cavity. The curing of the composition in the mould may allow the expanded rubber article to maintain the general shape of the mould cavity once released, and the approximate final size of the expanded rubber article may be controlled by the use of an appropriate amount of expansion agent. However, a frequent obstacle to the use of expanded rubber articles is the lack of precision in the shape and size of the expanded rubber article after release from the mould.
Often, it is desirable to bond solid rubber and expanded rubber materials to non-rubber substrates. However, durable bonds between such materials and substrates are difficult to achieve once the rubber has been cured.
Solid rubber materials may be bonded to non-rubber substrates using, for example, an elastomer bonding system and the following process. The part of the non-rubber substrate (for example a metal such as steel) to be bonded is coated with the elastomer bonding system comprising an elastomer bonding adhesive primer (EBAP) and an elastomer bonding adhesive (EBA). EBA's and EBAP's are typically mixtures of polymers, organic compounds and mineral fillers dispersed in organic solvent systems. In the next step, the coated substrate is contacted with a composition comprising an uncured rubber and a curing agent, and then the composition is cured. During the curing step, compounds in the elastomer bonding system and compounds in the composition diffuse between the materials and form crosslinking bonds between the cured rubber and the elastomer bonding system (as described in Bonding Elastomers: A Review of Adhesives and Processes, G. Polanski et al., iSmithers Rapra Publishing, 2004).
An obstacle to using the above solid rubber-substrate bonding process with expanded expandable rubber formulations is the difficulty of obtaining a strong and durable bond between the resulting expanded rubber part and the substrate. In particular, the rapid expansion of the expanded rubber upon release from the mould places large stresses on the expanded rubber-elastomer bonding adhesive interface, often inducing tear failure. Furthermore, the migration of gas generated by the expansion agent to the interface often promotes delamination.
It is also often desirable to bond expanded rubber materials to solid rubber materials. Solid rubber materials may also be bonded to other solid rubber materials using, for example, an EBA. However, the problems associated with bonding expanded rubber materials to non-rubber substrates also persist when attempting to bond expanded and solid rubber materials together.
A problem with conventional pneumatic tyres, for example pneumatic bicycle tyres, is their susceptibility to puncturing. Modern pneumatic tyres are typically a trade-off between puncture resistance, weight, durability, comfort and road grip. For example, durability and puncture resistance may be improved by using a harder material to form the tyre, but at the cost of road grip. Alternatively, a thicker layer of softer material may improve puncture resistance, but at the cost of weight. It will also be understood that a particular problem for cyclists is a need to carry a bicycle puncture repair kit/spare inner tube and a pump as precaution in case of getting a puncture, all of which add weight. Solid rubber tyres, which are not susceptible to puncture and offer similar or the same road grip as a pneumatic tyre made of the same material, typically suffer from problems of high weight and poor comfort. Recently, cast foam polyurethane materials have been used to make puncture resistant tyres of similar weight to traditional pneumatic tyres. Such products typically also benefit from being very low maintenance because, for example, it is often necessary to ‘top-up’ the air pressure in a pneumatic tyre at regular intervals. However, polyurethane foam tyres, especially polyurethane foam bicycle tyres, typically suffer from poor ride quality, grip and handling limitations, meaning that the tyres do not behave in the same way as a traditional pneumatic rubber tyre. It will be appreciated that an expanded rubber bicycle tyre should be able to cope with loads of up to 100 kg when installed on a bicycle wheel and provide 5 years or more and/or 5000 miles or more service without discernable and/or significant deterioration of its properties including it's shape, texture, hardness and grip, for example.
The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved process for preparing expanded rubber articles and to provide improved expanded rubber articles.